Cysteine is used for the purpose of, e.g., flavor improvement of foods. The conventional methods of producing cysteine include proteolysis, semi-synthesis etc., and the methods being mainly used at present are proteolysis and semi-synthesis.
When cysteine is to be used for the above purpose, there is strong need for food materials with a high content of cysteine (i.e., cysteine-rich food materials), but such food materials with a high content of cysteine are hardly known heretofore.
Indeed, there may be mentioned, as an example of accumulating γ-glutamylcysteine within yeast cells, an article wherein γ-glutamylcysteine synthetase was excessively expressed by a genetic engineering procedure, whereby γ-glutamylcysteine was accumulated in a maximal amount of 2% based on the dried yeast cells (Ohtake et al., Bioscience and Industry, vol.50, No.10, pp.989–994 (1992)). It is to be noted, however, that those results were obtained by the use of a yeast accumulating γ-glutamylcysteine in a larger amount, i.e., a laboratory model yeast made by a genetic engineering procedure which usually never happens in nature. Nor is there described in the article any idea of utilizing the yeast as a material for foods.
Further, it is known that γ-glutamylcysteine or a yeast extract or the like containing γ-glutamylcysteine when added to foods enhances “kokumi” (thickness, mouthfulness and continuity) of the food. Further, it is known that food materials obtained by heating γ-glutamylcysteine or a yeast extract or the like containing γ-glutamylcysteine in the presence of a sugar are effective in improving the flavors of foods, as described in, e.g., JP-A 4-66069 and JP-A 4-91762.
More specifically, the former (JP-A 4-66069) describes: “As a result of extensive study to obtain a more preferable meat flavor seasoning, the inventors have found that high-quality roast meat flavor-like seasonings excellent in stability and free of the above-described unpleasant odors and unpleasant tastes derived from yeast can be obtained by adding sugars and, as necessary, amino acids to a yeast extract containing 2 to 20% by weight (solid content) of sulfur-containing compounds such as glutathione, cysteine, or glutamylcysteine” (at the beginning of the “Means to Solve the Problem” section of the patent document), and on the basis of the findings, the inventors had completed “A process for producing seasonings, which comprises adding sugars and, as necessary, amino acids to a yeast extract containing a certain amount (2 to 20% by weight of the extract) of sulfur-containing compounds such as glutathione, cysteine, or glutamyl-cysteine and then heating the mixture at a temperature of 70 to 180° C. for 10 to 180 minutes in the absence of fats and oils” (the “claims” section of the patent document).
Further, the latter (JP-A 4-91762) describes: “Although there are a wide variety of existing meat flavors, all of them have different qualities from natural roast flavors of meat and there has been a need for an agent imparting flavors more similar qualities to natural roast meat flavors” (the “Problem to Be Solved by the Invention” section of the patent document); and “As a result of research on a wide variety of flavors resulting from heat-browning various amino acids and sugars for the purpose of solving these drawbacks, it has been found that a flavor composition having excellent roast meat flavors which could not be achieved heretofore can be produced by adding sugars to γ-glutamylcysteine, then dissolving the mixture in water, and heat-reacting the solution at a temperature of 70 to 180° C. for 10 to 180 minutes, and the present invention has been thereby completed” (the “Means to Solve the Problem” section of the patent document); and discloses “A process for producing a flavor composition, which comprises adding a sugar to γ-glutamylcysteine and heating the mixture at a temperature of 70 to 180° C. for 10 to 180 minutes.”
However, it has been noted that these flavor seasonings have the problem of burnt smells getting intensified with increased meat flavors.